Continuous-wave (CW) fiber lasers offer many advantages over other conventional diode pumped solid state (DPSS) lasers. Pulsed operation of such fiber lasers has many applications, but offers also some new challenges.
Generally, pulsed operation of a laser can be achieved by active or passive Q-switching or active or passive mode-locking. In Q-switching of a laser, a finesse parameter of the laser cavity is temporarily reduced by insertion of additional cavity loss, thereby allowing a population inversion in the laser's active medium to increase without an onset of lasing. When the loss element, typically an acousto-optic modulator (AOM), is switched off, an optical pulse quickly builds up within a few round trips of the cavity and a giant optical pulse is generated.
In conventional Q-switched lasers, round trip cavity loss of 5–10 dB is provided by an AOM positioned to deflect the laser mode into a first diffraction order and away from the cavity mirrors, while the optical output is provided by the zeroth diffraction order. Since this loss is typically higher than the cavity round trip optical gain, the AOM prevents lasing until it is switched off, at which point a giant optical pulse is generated within the cavity.
U.S. Pat. No. 5,193,096 to Amano discloses an acousto-optic Q-switched solid state laser having a Q-switching acousto-optic element intervening between the pair of laser resonance mirrors for controlling Q-switching oscillation by abruptly changing a loss of resonant laser light passing therethrough.
A similar approach to Q-switching was used in Q-switching of several prior-art fiber lasers. U.S. Pat. No. 4,955,025 to Mears et al discloses a diode-pumped fiber laser which can be Q-switched using an intra-cavity acousto-optic deflector in a transmission mode or a mechanical chopper, and having an output mirror partially transmissive at the lasing wavelength. U.S. Pat. No. 5,818,630 to Fermann et al. discloses a multi-mode fiber laser wherein an optical switch is employed for output coupling which also serves to modulate the loss (Q) of the cavity defined by two end mirrors. Alternatively, a pulsed output of this laser can be extracted by using a partially transmissive mirror.
However, in a fiber laser, especially a double-clad fiber laser, the length of the active medium is typically much longer than in a DPSS laser or other types of lasers, resulting in a round trip optical gain in excess of 10 dB for a cw fiber laser under typical pumping conditions, and a possibility of a much higher, up to 50 dB and more, round trip small signal gain for Q-switched lasers. In this case, maximum attainable loss from an AOM operating as an aforedescribed zero-order Q-switch can be insufficient to hold off the onset of lasing, and therefore this approach cannot always be used.
Instead, if an AOM is used as a Q-switch, the laser must be Q-switched in a first order operation wherein a laser cavity is formed when the AOM is switched on and the laser mode is deflected to the output cavity mirror in the first or higher diffraction order. The key advantage of the first order operation is that a contrast ratio of cavity loss in an “on” and “off” state in this instance can be more than 50 dB compared to less than 10 dB that can be typically achieved in the zero-order operation.
U.S. Pat. No. Re29,421 to Scott discloses first-order Q-switching with electronically selectable gain in a laser system comprising an acousto-optic deflector. A laser cavity includes a rod of lasing material between first and second reflecting means, said second reflecting means being positioned along a line that forms a preselected angle with the longitudinal axis of said laser rod; an acousto-optic beam deflector which deflects laser emission to said second reflecting means.
Other prior-art solutions for generating optical pulses in a fiber laser without an AOM have also been discussed. U.S. Pat. No. 6,510,167 to Jain et al. discloses a mode-locked fiber laser employing a fixed or tunable FBG at one end and an electro-optically tunable FBG at another end of an active fiber section, wherein the electro-optically tunable FBG is modulated to achieve active mode-locking. Relatively small modulation depth of the electro-optical tuning of the FBG described in U.S. Pat. No. 6,510,167 prevents however efficient Q-switching. U.S. Pat. No. 5,444,723 to Chandonnet et al discloses an optical switch in Q-switched fibre laser having length of exposed optical fibre mounted substantially parallel with movable index overlay perturbation pad whose refractive index is greater than optical fibre core for controlling amount of light escaping from core. Exposing a core of an optical fiber can however have negative effect on its long-term reliability and should generally be avoided.
The aforedesribed prior art Q-switching solutions, although answering their respective purposes, in addition to their aforementioned limitations have a common feature that negatively affects their output efficiency: the laser pulses are outputted when their respective Q-switches are turned on, and the laser is in a low-loss, high-Q state of the cavity. Therefore the pulses have to be output either through a partially transmitting mirror or through a beam-splitter, thereby reducing their output optical power. It will be therefore advantageous to have a laser apparatus for Q-switching having a partially reflecting output coupler that provides a high contrast ratio and reduces its reflectivity to output laser pulses.
It is an object of present invention to provide a pulsed fiber laser for efficient generation of high-power optical pulses and comprising a controllable reflecting/transmitting output coupler having a high contrast ratio and a switching time less than a pulse roundtrip in the laser cavity, and wherein the controllable reflecting/transmitting output coupler is switched to a transmitting state for outputting optical pulses.
It is another object of this invention to provide a method of Q-switching of a fiber laser enabling efficient generation and control of high-power optical pulses.